The passenger boarding bridge is used in airports and is very popular due to its convenience and security. The boarding bridge assists a passenger when the passenger is boarding an airplane directly from a terminal building. At present, many kinds of passenger boarding bridges are disclosed, such as in Chinese patent No. ZL95226673.3, ZL00258374.7 and American U.S. Pat. No. 5,855,035. Conventionally, there are two manners for supporting a beam of a boarding bridge, which is respectively named a single-point supporting and a double-point supporting. With reference to FIG. 1, a passenger boarding bridge 100 in so-called single-point supporting manner is disclosed in Chinese Patent ZL00258374.7, which comprises a rotunda connected to a terminal (not shown), a tunnel 101 which can be retracted or extended to change its length, elevation system used for adjusting the height of the tunnel 101, and a wheel mechanism 102 for supporting the tunnel 101 through the elevation system etc. The wheel mechanism 102 is provided with two wheels attached respectively to both ends of a beam. A bearing plate is substantially located in the center of the beam and used for supporting the elevation system. However, the bridge 100 has poor stability, as the tunnel 101 often wobbles when it carries passengers or when the wheel mechanism 102 is driven to move or turn around the bearing plate. Chinese patent ZL95226673.3, as shown in FIG. 2, discloses another bridge 200 with a wheel mechanism 201 for supporting an elevation system 203 which can adjust or control a tunnel 202 to change its height. Now referring to FIG. 2 and FIG. 3, specially, two sets of wheel assembly 204 are attached to both ends of a beam 301 to improve the stability of the bridge 200, each set of which further comprises a revolving base 302 used for supporting, a hinge support 303 and a hinge shaft 304, and two wheels 305. Importantly, for this kind of bridge 200 in so-called double-supporting manner, a synchronous steering mechanism 307 is necessary in order to drive the wheel assembly 204 at two ends of the beam 301 to move and rotate, or a control system is alternatively utilized to drive the wheel 305 in order to accomplish synchronous moving and/or rotation of the wheel 305. In fact, the conventional boarding bridge 200 according to the double-supporting manner is more stable than before, while it has a complex structure and a low safety, and the control system for synchronous operation of the wheels are even more complicated and expensive. Furthermore, the reliability of the bridge is deduced accordingly as it is unavoidable for the control system to break down.